Metal vapor generators

ABSTRACT

A metal vapor generator comprising: a hollow tube containing a metal vapor releasing substance, the tube having an opening to permit escape of the metal vapor; a terminal having a cross section of the same size and geometric shape as the cross section of the tube; and a retainer holding the end of the terminal against the end of the tube; whereby the metal vapor releasing substance is held within the tube. The generator can optionally have a flexible member for temperature compensation. These generators are useful for releasing metals such as cesium in tubes such as image intensifiers.

itgd States Patent Poi-ta et a1.

[451 Jan. 18, 1972 [541 METAL VAPOR GENERATORS [72] lnventors: Paolo Della Ports; Carlo Emili, both of [21] Appl. No.: 856,567

[30] Foreign Application Priority Data Sept. 13, 1968 Italy ..21175 A/68 [56] References Cited 3,058,842 10/1962 Kahan et al. ..l18/49 X 3,235,646 2/1966 Sens ..2l9/427 X 3,387,116 6/1968 Dupuis ..l3/2S X 3,408,608 11/1968 Chikinev ..338/3l6 Primary Examiner-C. L. Albritton Attomey -David R. Murphy [57] ABSTRACT A metal vapor generator comprising: a hollow tube containing a metal vapor releasing substance, the tube having an opening to permit escape of the metal vapor; a terminal having a cross section of the same size and geometric shape as the cross section of the tube; and a retainer holding the end of the terminal against the end of the tube; whereby the metal vapor releasing substance is held within the tube. The generator can optionally have a flexible member for temperature compensation. These generators are useful for releasing metals such as cesium in tubes such as image intensifiers.

19 Claims, 3 Drawing Figures PATENTEU M18127: 3.636302 sum 1 [IF 2 PATENTEB .mu 3 m2 SHEET 2 [IF 2 METAL VAPOR GENERATORS Metal vapor generators are well known in the art as described, for example, in Italian Pat. No. 784,903 and in Eichenbaum, et al., Cesium Vapor Dispenser, The Review of Scientific Instruments, Volume 35, No. 6, June 1964, pages 691-693. These generators can be used to produce vapors of alkali metals in electronic tubes to produce photosensitive surfaces such as those in television pickup tubes, photomultiplier tubes, image intensifier tubes, and other types of vacuum tubes. These prior generators are commonly manufactured by filling a tube with a metal-vapor-releasing substance, and then closing the same at each end for instance by crimping the tube, and partially removing the filling substance. Such a method of manufacture is expensive and frequently results in damage to the metal-vapor-releasing substance which can cause loose particles. These loose particles cause difiiculties within the tube and may ruin the photosensitive surface. Another disadvantage of these generators in connection with the temperature gradient which results along the length of the tube during the activation process: this temperature gradient causes, in the region adjacent to the ends, insufficient activation of the metal-vapor-releasing substance. Yet another disadvantage is the change in length of the tube which results in lateral displacement of the terminals causing them to break loose from their housing. When the housing constitutes the walls of a vacuum tube, the result can be a partial or complete loss of vacuum. 1

Accordingly, it is an object of the present invention to provide metal vapor generators substantially free of one or more of the disadvantages of prior generators.

Another object is to provide generators free of loose particles both before and after metal vapor release.

Yet another object is to provide generators which can be more economically manufactured than prior generators.

Still another object is to provide generators exhibiting an even temperature along the length of the tube.

An additional object is to provide metal vapor generators having a compensation of thermally induced stresses in the length of the tube.

Additional objects and advantages will be apparent to those skilled in the art by reference to the following detailed description of the present invention and drawings wherein:

FIG. 1 is a partially exploded isometric view of one embodiment of the present invention;

FIG. 2 is an isometric exploded 'view of one end of the generator of the present invention having a flexible member for stress compensation; and I FIG. 3 is a schematic representation of the use of generators of the present invention in a vacuum tube.

According to the present invention, there is provided a metal vapor generator comprising: a hollow tube containing a metal-vapor-releasing substance, the tube having an opening to permit escape of the metal vapor; a terminal having a cross section of the same size and geometric shape as the cross section of the tube; and a retainer holding the end of the terminal against the end of the tube; whereby the metal-vapor-releasing substance is held within the tube.

When the generator has a terminal in each end, it can optionally have a flexible member comprising a curved strip attached to the terminals to provide temperature compensation.

The hollow tube is generally formed of a single sheet of metal which is preferably a metal of high ohmic resistance. This sheet of metal is arcuately bent to form a tube having a cross section of any regular or irregular geometric shape, such as round or hexagonal but preferably trapezoidal. The tube contains and is preferably filled with a metal-vapor-releasing substance. Any metal-vapor-releasing substance employed in prior generators can be employed. In general, the metalvapor-releasing substance can be any material or mixture of materials which, upon heating, releases the desired metal; however, in a preferred embodiment of the present invention, the substance is a mixture of a particulate alkali metalchromate and a stoichiometric excess of a particulate reducing agent for the chromate. Examples of alkali metal chromates include, among others, cesium chromate, potassium chromate, and sodium chromate. In another embodiment of the present invention, the substance is a mixture of a reducing agent and a reducible compound of mercury such as mercuric oxide. The substance is in particulate form such as that which passes through a screen of 270 mesh/inch and is retained on a screen of 600 mesh/inch. However, to reduce the danger of loose particles in an electronic tube, the particulate substance is compressed into a coherent form.

The generator is provided with at least one, and preferably two, terminals having a cross section of the same size and geometric shape as the crom section of the tube. A retainer provides means for holding the ends of each terminal against the end of the tube in order'to hold the metal-vapor-releasing substance within the tube. In a preferred embodiment the retainer is a band which substantially encircles the tube and the terminals at their plane of contact.

Referring now to the drawings,and in particular to FIG. 1, there is shown a preferred embodiment of the present invention in the form of a generator 10 comprising a tube 11, terminals 12 and 13, and retainers 14 and 15. The tube 11 is formed of a single sheet of metal of high ohmic resistance and comprises a base 16, two outwardly slanting walls 17 and 18, each of which is respectively connected to a top wall 19 and 20. Top walls 19 and 20 meet in juxtaposed relationship defining an opening 21. The base 16 and the walls l7, 18, 19 and 20, together define a chamber which is filled with a metalvapor-releasing substance 22 which is a mixture of one part by weight of Cs,CrO and two parts by weight Si. The tube 11 can be of any suitable length but preferably has at least one planar end which is preferably perpendicular to the axis of the tube 11.

Terminal 12 has a butting portion 23 and a supporting portion 24. The butting portion 23 has a base 25, two outwardly slanting sides 26 and 27, and a top 28. The cross section of the butting portion 23 has the same size and geometric shape as the cross section of the tube 11. The butting portion 23 has a planar end 29 measured in a plane perpendicular to the axis of the terminal 12. 4

The retainer 14 is in the form of a band having a top 30 and two depending sides 31 and 32 terminating respectively in tabs 33 and 34. The total length of the band 14 is preferably slightly less than the circumference of the tube 1 1.

To construct the generator 10 a filled tube 11 of indefinite running length is cut in a plane perpendicular to the axis of the tube 11 by any suitable means such as a high-velocity cutting wheel. Referring to FIG. 1, the terminal 12 is moved axially towards the tube 11 until the face 29 contacts the planar end of the tube 11. The retainer 14 is then moved downwardly until the top 30 of the retainer 14 rests upon the top 28 of the butting portion 23, as well as a portion of the top walls 19 and 20 of the tube 11. The sides 31 and 32 of the retainer 14 are bent inwardly until'they contact the walls 17 and 18 of the tube 11 and the sides 26 and 27 of the terminal 12. The tabs 33 and 34 are then bent until they almost meet and are parallel to the base 16 of the tube 11 and the base 25 of the butting portion 23 such that the band 14 holds the end of the tube 11 against the end of the terminal 12. The retainer 14 is then secured in place by any suitable means such as spot-welding. When constructed in this manner, the terminal 12 and the retainer 14 have the appearance of the terminal 13 and retainer 15.

Referring now to FIG. 2 wherein like numerals designate like parts, there is shown an embodiment of the generators of the present invention further comprising a flexible member which provides thermal compensation. The flexible member is preferably of a material of high ohmic resistance and comprises an S-shaped strip 40 having a width w substantially equal to the transverse dimension of the butting portion 23 measured across the top 28 from side 26 to side 27. The length of the strip 40 is substantially greater than the width w. One planar side 41 of the strip 40 is attached to the butting portion 23 of the terminal 12 at a pointnear the end of the strip 40.

The other planar side 42 of the strip 40 is attached to a support 43 which also preferably functions as an electrical lead. The point of attachment of the side 42 is near the end of the strip 40.

In the embodiments depicted in FIGS. 1 and 2, the tube 1 l, retainers l4 and 15, terminals 12 and 13, and strip 40 are all manufactured of a high ohmic resistance alloy of 80 percent nickel and 20 percent chromium.

Operation of the generator is illustrated by reference to FIG. 3 wherein there is schematically shown a glass vacuum tube 50 such as an image intensifier tube. The generator 10 is mounted within the vacuum tube 50 and has terminals 43 and 43' extending through the walls of the tube 50. Terminals 43 and 43 are connected in series by conductors 51 and 52 to a source of electricity shown schematically as generator 53. The circuit is provided with a switch 54 in series. In operation, the tube 50 is evacuated by any known means, whereupon the switch 54 is closed causing current to flow through the terminals 43 and 43', the strips 40 and 40', the retainers l4 and 14', and the tube 11. Due to the high-ohmic resistance, the walls of the tube 11 heat, which in turn heats the substance 22 within the tube 11. When the temperature of the substance 22 has reached about 1,000 K, the silicon reacts with the cesium chromate releasing cesium vapor which escapes from the substance 22 out of the opening 21 shown in FIG. 1.

The electrically induced heating of the tube 11 causes the dimension I to increase; however, due to the flexible nature of the strips 40 and 40, the distance 1, remains constant which avoids damage to the seal between the terminals 43 and 43 and the walls of the tube 50. In a preferred embodiment, the flexible members 40 and 40' each have a total ohmic resistance such that, when passing current through them and the tube 11, the temperature of the members 40 and 40' is substantially equal to the temperature of the tube 11, thus eliminating undesirable thermal gradients in the tube 11. The total ohmic resistance of the members 40 and 40' can be controlled by appropriate choice of materials and cross-sectional area according to wellknown electrical engineering principles.

Although the invention has been described in considerably detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described above and as defined in the appended claims.

What is claimed is:

1. A metal-vapor-generating means comprising:

A. a hollow tube containing a particulate metal-vaporreleasing substance compressed into a coherent form, the tube being closed in sufficiently to restrict escape of the particulate substance in all orientations and attitudes of the tube and leaving an opening to permit escape of metal vapor released by the substance, the tube having two planar ends of a given geometric shape;

B. terminals each having a planar end of the same size and geometric shape as the planar ends of the tube; and

C. a retainer band holding the planar end of a tenninal against a planar end of the tube;

whereby particles from the particulate metal-vapor-releasing substance are prevented from leaving the tube both before and after metal vapor release.

2. The generating means of claim 1 wherein the tube, the terminal and the retainer band are all constructed of the same high ohmic resistance metal.

3. The generating means of claim 1 wherein the tube and the terminal have a trapezoidal cross section.

4. The generating means of claim 1 wherein the retainer band substantially encircles the tube and the terminal.

5. The generating means of claim 4 wherein the retainer band has ends thereof which meet on the side of the tube opposite the opening of the tube.

6. The generating means of claim 1 wherein the retainer band is welded to both the tube and the terminal.

7. The generating means of claim 1 wherein said substance releases mercury vapor when heated.

8. The generating means of claim 1 wherein the metal is an alkali metal.

9. The generating means of claim 8 wherein the alkali metal is cesium.

10. The generating means of claim 8 wherein the metalvapor-releasing substance is a mixture of a particulate alkali metal chromate and a stoichiometric excess of a reducing agent for the chromate.

11. A vacuum tube containing the generating means of claim 1 and having walls fixedly attached to said terminals by flexible members.

12. A metal-vapor-generating means comprising:

A. a hollow tube of high ohmic resistance conductive material containing a particulate metal-vapor-releasing substance, the tube having an opening to permit escape of metal vapor,the tube having a trapezoidal cross section and planar ends;

B. temiinals of high ohmic resistance each having a planar end of a trapezoidal cross section of the same size and shape as the trapezoidal cross section of the tube, a planar end of each terminal contacting a planar end of the tube; and

C. a band substantially encircling the tube and the terminal at their plane of contact, the band holding the end of the tube against the end of the terminal.

13. A metal-vapor-generating means according to claim 1,

further comprising:

a flexible member comprising a curved strip attached to each of the terminals, each flexible member being attached to a support;

whereby heating of the tube causes a thermally induced change in the length of the tube while maintaining constant the distance between the supports.

14. A metal-vapor-generating means according to claim 13 wherein the tube, the terminals and the flexible member are all constructed of high ohmic resistance conductive material, whereby said heating can be caused by passing an electrical current through the tube.

15. The generating means of claim 14 wherein the flexible member has a total ohmic resistance such that when passing current through the member and the tube, the temperature of the member is substantially equal to the temperature of the tube.

16. A vacuum tube containing the generating means of claim 14 and having walls fixedly attached to the flexible members.

17. The generating means of claim 13 wherein the flexible member is S-shaped.

18. A metal-vapor-generating means comprising:

A. a hollow tube containing a particulate metal-vapor releasing substance compressed into a coherent form, the tube being closed in sufficiently to restrict escape of the particulate substance in all orientations and attitudes of the tube and leaving an opening to permit escape of metal vapor released by the substance, the tube having planar ends of a given geometric shape;

B. terminals each having a planar end of the same size and geometric shape as the planar end of the tube;

C. a retainer band holding the planar end of each terminal against a planar end of the tube; and

D. a flexible member of high ohmic resistance comprising an S-shaped strip having a width substantially equal to the transverse dimension of the terminal and having a length greater than its width, one planar side of the strip being attached to the terminal at a point near the end of the strip, the other planar side of the strip being adapted to be attached to an electrical lead at a point near the opposite end of the strip.

19. A metal-vapor-generating means adapted for use with first and second sources of electrical potential for obtaining metal vapor by electrically heating a particulate, vapor-releasing substance, comprising:

A particulate, vapor-releasing substance compressed into a coherent form of said first and second planar ends, and each said terminals being adapted to be connected to a respective one of said sources of electrical potential for electrically heating the tubular means and the substance therein, and

D. retainer band means for holding the respective planar surfaces and the respective planar ends abutting sufi'iciently closely to prevent the dispersal of any particles of said particulate substance from said planar ends. 

2. The generating means of claim 1 wherein the tube, the terminal and the retainer band are all constructed of the same high ohmic resistance metal.
 3. The generating means of claim 1 wherein the tube and the terminal have a trapezoidal cross section.
 4. The generating means of claim 1 wherein the retainer band substantially encircles the tube and the terminal.
 5. The generating means of claim 4 wherein the retainer band has ends thereof which meet on the side of the tube opposite the opening of the tube.
 6. The generating means of claim 1 wherein the retainer band is welded to both the tube and the terminal.
 7. The generating means of claim 1 wherein said substance releases mercury vapor when heated.
 8. The generating means of claim 1 wherein the metal is an alkali metal.
 9. The generating means of claim 8 wherein the alkali metal is cesium.
 10. The generating means of claim 8 wherein the metal-vapor-releasing substance is a mixture of a particulate alkali metal chromate and a stoichiometric excess of a reducing agent for the chromate.
 11. A vacuum tube containing the generating means of claim 1 and having walls fixedly attached to said terminals by flexible members.
 12. A metal-vapor-generating means comprising: A. a hollow tube of high ohmic resistance conductive material containing a particulate metal-vapor-releasing substance, the tube having an opening to permit escape of metal vapor, the tube having a trapezoidal cross section and planar ends; B. terminals of high ohmic resistance each having a planar end of a trapezoidal cross section of the same size and shape as the trapezoidal cross section of the tube, a planar end of each terminal contacting a planar end of the tube; and C. a band substantially encircling the tube and the terminal at their plane of contact, the band holding the end of the tube against the end of the terminal.
 13. A metal-vapor-generating means according to claim 1, further comprising: a flexible member comprising a curved strip attached to each of the terminals, each flexible member being attached to a support; whereby heating of the tube causes a thermally induced change in the length of the tube while maintaining constant the distance between the supports.
 14. A metal-vapor-generating means according to claim 13 wherein the tube, the terminals and the flexible member are all constructed of high ohmic resistance conductive material, whereby said heating can be caused by passing an electrical current through the tube.
 15. The generating means of claim 14 wherein the flexible member has a total ohmic resistance such that when passing current through the member and the tube, the temperature of the member is substantially equal to thE temperature of the tube.
 16. A vacuum tube containing the generating means of claim 14 and having walls fixedly attached to the flexible members.
 17. The generating means of claim 13 wherein the flexible member is S-shaped.
 18. A metal-vapor-generating means comprising: A. a hollow tube containing a particulate metal-vapor-releasing substance compressed into a coherent form, the tube being closed in sufficiently to restrict escape of the particulate substance in all orientations and attitudes of the tube and leaving an opening to permit escape of metal vapor released by the substance, the tube having planar ends of a given geometric shape; B. terminals each having a planar end of the same size and geometric shape as the planar end of the tube; C. a retainer band holding the planar end of each terminal against a planar end of the tube; and D. a flexible member of high ohmic resistance comprising an S-shaped strip having a width substantially equal to the transverse dimension of the terminal and having a length greater than its width, one planar side of the strip being attached to the terminal at a point near the end of the strip, the other planar side of the strip being adapted to be attached to an electrical lead at a point near the opposite end of the strip.
 19. A metal-vapor-generating means adapted for use with first and second sources of electrical potential for obtaining metal vapor by electrically heating a particulate, vapor-releasing substance, comprising: A particulate, vapor-releasing substance compressed into a coherent form B. an elongated, partially opened tubular means for containing the substance sufficiently closely to restrict escape of particles of the substance through the partial opening and sufficiently loosely to allow the dispersal of metal vapor released by the substance, the tubular means having first and second planar ends and being constructed of a high-resistance electrical conductor C. first and second conductive terminals each having one planar surface situated abutting against a respective one of said first and second planar ends, and each said terminals being adapted to be connected to a respective one of said sources of electrical potential for electrically heating the tubular means and the substance therein, and D. retainer band means for holding the respective planar surfaces and the respective planar ends abutting sufficiently closely to prevent the dispersal of any particles of said particulate substance from said planar ends. 